Frequency modulator



Oct} 10, 1950 s. M. RODHE 2,524,992

FREQUENCY MODULATOR- Filed March 23, 1948 3 Sheets-Sheet 1 svEN MAGNUS RODii}; I

Cl TORALEYs V Oct. 10, 1950 s. M. RODHE 2,524,992

FREQUENCY monum'oa Filed March 23, 1948 3 Sheets-Sheet 2 INVNTOR SVEN MAGNUS RoDH}:

Oct. 10, 1950 s. M. RQDHE 2,524,992

I FREQUENCY MODULATOR Filed March 23, 1948 3 Sheets-Sheet 3 Jso INVEINTOR svEN MAGNUS RQDHE ATTORNE Y5 Patented Oct. 10, 1950 TENT. OFFICE .FREQUEN CY MODULATOR Sven Magnus Ro dhe, Stockholm, Sweden, assignor to Telcfonaktiebolaget L M Ericsson, Stockholm, Sweden, a company of Sweden Application March '23, 194-8,-Serial No. 16,530 In Sweden March 27, 1947 p .7 Claims.

.Thisrinvention-relates to a novel device for frequencytmodulation' of carrier transmitters. The frequency deviation obtained ,in this modulator may ;be directly utilized for the final transmitter frequency without afollowing chain of frequency multiplicators as the case is for modulators with reactance tubes, now mostfrequently in 'use.

' For'this'purpose 'itis' also knownto utilize rectifiers toootain a-variable couplinghetween an osciilating circuit, the frequency of which hasto be vai ied in depen'dence of a signal intensity, and a second circuit having an influence on the osoill-ating frequency of :the 'first mentioned circuit. See for instance an article lry 'BrahihaH and Bdughfm'o'od, Frequency Modulated Carrier Telegraph System, in I. E. Transactions, ppL 36-39, 3anuary, i942, especialIy Fig. "7.

This inventiondiffers however from those previously known by 'one or more of 'the following features.

The modulator parts are galvanically-entirely isolated from't-he-vital partsof -theosciilator.

The circuits-having an' infiuence on the frequency of the oscillating circuit are separated from the signal voltage source by aid of rectifiers arranged symmetrically in'such a way thatundue influence from said source on the circuits is prohibited. The -diiferent parts of thesecircuits are separatedfrom each other in such a way that the trimming of the modulating properties of the circuits may easily be made without noticeable influence from one circuit on the other.

The modulator according to the invention contains an oscillator with an oscillating circuitto which two diilerent circuits are coupled, in a preferred embodiment of the invention inductively, but otherwise in any manner known per se. These-circuits contain each one resistance, variablein dependence of the signal voltage, with which the-oscillator carrier frequency is to be modulated-in such a way that the resistance for one polarity of the signal voltage is practically nonconducting and for the opposite polarity of the signal voltage has ,.a.resistance value decreasing with increased signal voltage. One of the circuits is provided .with circuit elements whichcause anincreaseof the frequency of the oscillating'circuit when the resistance value decreases andthe other circuit with elements causing-a decrease of the frequency in the same case. One may for instance contain an inductance in series-with a coil coupled to the oscillating circuit and the other may contain a capacity in-series with a corresponding *coil. Also more complex impedances may be used for the purpose. The variable resistances of the difierentcircuits are connected tothe signal voltage source .in such a way that they are conducting for opposite po iarities cf the, signal voltage.

According to theinvention the resistancesyariable in themanner described vconsist in rectifier arrangements connected as more closely described in connection with the figures. i

:Fig. l shows azschematic diagramof a carrier transmitter according totheinvention.

rigor J shows a curve illustrating theresistance value of the resistances inserted in thelcircuits,

having an influence on the carrier frequency,-and

according to Fig. lconsisting inrectifienbridges. Fig. 3 illustrates the type .of relation between the oscillator frequency and this resistance .value.

Fig. 4 showsa simplified circuit illustrating the relationaccordingto Fig. 3. ,rFig. 5 illustrates theoscillator frequencyLas a function of the signalcurrcntfor an embodiment. of'the invention.

- Fig. 6 isa diagram illustrating a detail .of

modulator, somewhat difiering .from corresponding part of Fig. .1, showing .rhowa certain .threshold value for the signal voltage before a modu lation is obtained is introduced into the circuit and Fig. 7 the relation between the oscillator fre-" quency and the signal currentina device according to Fig. 6.

The carrier transmitter consists of the amplifier F, to the input terminals 5 and E of which an oscillating circuit is connected consisting of an inductance-coil Lo and a condenser C0. Positive back feed is arranged from the terminal '3 of the output terminals v3 and 4 to the oscillating-circuit to such a degree that the system will self-oscillate. In Fig. 1 this isillustrated .by a resistance R5.

The 'coil Lois provided with two .difie'rentsecondary windings Loi and L02. A condenser C is connected inseries with the .alternating current diagonal A-B of a rectifier vbridge LE2 to the winding Lo! and ran .ind'utance coil 'L .is in the same manner connected in series with the alternating currentdiagonal A.B of a second rectifier .brid-ge LE! to the winding L02. The con denser C and the resistance B. have no ..other function but forthe trimmingtof .the modulator.

ihe direct .current diagonals .CD hi the rectifiersLBl and LR2.are mutually connected in parallel ..to .a resistance Rd carrying the signal current avith which the .transmitter will-be frequency modulated. Resistances R! .and .R2 as well as .a resistance R3 are .connected inseries with the rectifier bridge diagonals in order to' minimize the influence of one circuit on the other. The inductance coils Li'and the condenser CI constitute a low pass filter, to which the resistance R4 .is-the .main-terminating-resistance. Ihe

3 signal voltage by which the transmitter has to be modulated is connected to the input terminals l and 2.

A signal voltage with such a polarity that the input terminal I is positive in relation to 2 results in a current through the rectifier bridge LRZ in the direction C to D and through the resistances R2 and R3. The rectifier bridge LRI, the direct current diagonal of which is connected in opposite polarity to the corresponding diagonal of the rectifier bridge LRZ, receives back voltage without a current transit. For the opposite polarity of the signal voltage, i. e. terminal -2 positive in relation to terminal I, the direct current diagonal CD of the rectifier bridge LRI carries current and the same diagonal of the rectifier bridge LE2 receives back voltage without current.

The resistance Rw (total resistive effect) in the alternating current diagonal AB of a rectifier bridge is dependent on the current Is through the direct current diagonal CD as shown in Fig. 2. At zero-current the resistance Rw is very great, but for an increased current it will sink rapidly for small values of Is and thereafter slowly approach very low values for higher currents. For opposite voltage, i. e when D is positive in relation to C, Rw is very great. As the resistances of the rectifier parts AC Cl3, B-D and D-A in the bridge have approximately the same value, the points C and D will have the same potential for an alternating current through the alternating current diagonal AB. The alternating current circuit is thus entirely de-coupled from the control circuit of the transmitter.

For the signal voltage zero the resistances AB of the two rectifier bridges thus are very great and the impedances of the inductance L and the capacitance C have consequently no influence on the oscillating circuit LoCo. The carrier frequency transmitter thus oscillates with the frequency f0, determined by Lo and C0, and from the output terminals 3 and 4 voltage is obtained with this frequency.

For a certain signal voltage, which gives a positive voltage to the input terminal I, the resistance between the points AB of the rectifier LE2 decreases as the voltage is increased. The capacitance C is therefore coupled tighter in parallel to the oscillating circuit LoCo via the winding Lol and the frequency of the transmitter will sink 1 from the initial value f0, in dependence of the magnitude of the signal voltage, applied to the input terminals. The rectifier bridge LRI has a high resistance between the points AB and thus the inductance L has no influence on the frequency.

A negative signal voltage on the terminal I, however, will cause a decrease of the resistance between the points AB of the rectifier bridge LRI while the resistance of the rectifier bridge LR2 is high. The inductance L is therefore connected in parallel to the oscillating circuit via the winding L02. The frequency will consequently rise with increased signal voltage of this polarity.

Examples of the frequency deviation is shown in Fig. 3, where an inductance L, according to Fig. 4 in series with a resistance Rw, is connected in parallel to an oscillating circuit Lo, Co. The resonance frequency fr is obtained when the impedance Z of the circuit is real. In Figure 3 the relation where in is the resonance frequency for L and 4 Co alone, has been shown as a function of the resistance value Rw. For Rw= this relation is 1. For a decreased value of Rw the frequency fr, for the value of L used in the example, will increase to 1.20 f0 for Rw=0. A corresponding curve is obtained for anoscillating circuit with a capacitance C connected in parallel to the circuit in series with a resistance Rw. In this case, however, the relation fr/fo will be less than one.

In Fig. 5 the frequency of the transmitter is shown as a functionof the control current Is from the input terminal I to the input terminal 2. The lower frequency fl is a frequency which would be obtained when Rw=0 in the rectifier bridge LR2 and I2 is the frequency to be obtained when Rw=0 in the rectifier bridge LRI.

The symmetry of the curve may be adjusted by aid of the resistances RI and R2 of Fig. 1. The maximum deviation inside the range fl to i2 is set by means of resistance R3. In the shown example the usable range of frequencies where the curve is straight extends between the frequencies fl and f;

The resistance RI (Fig. 1) is used for straightening the curve in the neighbourhood of the average frequency f0, where the control current is low.

The condenser Cl is used for trimming of the inductance L.

The low pass filter LICI is used for limiting the signal frequency band.

An amplitude limiter may be connected after the output terminals 3 and 4 in order to eliminate undesirable amplitude modulation of the carrier or may be included in the amplifier F. s

In Fig. 1 and in the preceding description a shunt connection of simple inductances L and capacitances C with the oscillating circuit has been shown. In certain cases it may be suitable to use more complicated impedances.

If the signal line connected to the input side of the frequency modulator carries background interference it may be suitable to connect a bias voltage U to the rectifiers, according to Fig. 6 in series with resistances RB and R1 between point C on the rectifier bridge LRI and point D on the rectifier bridge LRZ. The polarity of the bias voltage is such that the two rectifiers receive back voltage. Hence a certain current Iso is necessary before a frequency deviation takes place. A modulation curve with such bias connected to the rectifiers is shown in Fig. 7.

I claim:

1. A frequency modulation system for a source of carrier voltage comprising in combination with a frequency determining circuit of said carrier voltage of an inductance, a capacitance, a pair of four element rectifier bridges, a circuit connecting one diagonal of one of said bridges to said inductance, a circuit connecting one diagonal of the other bridge to said capacitance, separate means coupling each of the last mentioned circuits to said first mentioned circuit, each of said diagonals being opposite to one in which the bridge elements are all conducting from one end to the other thereof, input terminals for said modulator and means connecting the remaining diagonals of said bridges in parallel to said terminals in such a manner that one diagonal only is conducting for each polarity of a signal applied to said terminals.

2. A frequency modulation system for a source of carrier voltage comprising in combination with a frequency determining inductance of said carrier voltage, of a pair of coils inductively coupled to said inductance, a pair of four element rectifier bridges each having the elements arranged to be all conducting from opposite ends of a first diagonal, an inductive reactance, means connecting said reactance, one of said coils and the second diagonal of one of said bridges in series, a capacitive reactance, means connectin said second reactance, the other coil and the second diagonal of the remaining bridge in series, means connecting the first diagonals of said bridges in parallel but oppositely directed as to conductivity and means to apply a modulating signal of reversing polarity to said last mentioned means.

3. The system as claimed in claim 2 in which means is provided to regulate the eifect of one of said reactances on said inductance.

4. The system of claim 2 in which said inductive reactance is shunted by a variable condenser 20 and its bridge diagonal is shunted by an adjustable resistance.

5. The system of claim 1 in which a separate adjustable resistance is connected between one end of each of the remaining diagonals and one of the said input terminals, and an adjustable re sistance is connected between the other terminal and the junction of the opposite ends of said diagonals.

6. The system of claim 2 in which a source of direct current potential is connected to the said bridges to provide back voltage to both of the second diagonals.

7. The system of claim 2 in which said last mentioned means includes a low pass filter and a terminating resistance across which the modulating signal is impressed.

SVEN MAGNUS RODHE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,045,107 Shore June 23, 1936 2,374,000 Crosby Apr. 1'7, 1945 2,397,992 Stodola Apr. 9, 1946 

